Field bus systems are digital communication systems widely used in industry, which connect a multiplicity of field devices such as sensors, actuators and drives, input and output assemblies, controllers, operating devices and displays and other components of automation technology with one another. At present, different field bus systems having different characteristics are established on the market, e.g. PROFIBUS, INTERBUS, Control Net, FOUNDATION field bus, CAN or LON. In recent years, Ethernet-based communication systems with an extension for real time, which have the potential of replacing the previously known field bus systems in the future, have been increasingly becoming established in the industrial domain. In the context of the present invention, the term field bus thus designates such local networks which are suitable for industrial applications for connecting field devices and controllers.
FIG. 1 schematically shows an example of such a field bus system in which a central controller 10 is connected to a multiplicity of field devices 12, 14, 16, 18 via a field bus line 20. The invention can also be applied to systems having a number of controllers.
A field bus system operates on the basis of a protocol stack which is built up of three to seven protocol layers building upon one another, including a physical layer, a data link layer and an application layer. The data transmission between the field devices occurs on the physical layer and the data link layer. The physical layer specifies how signals are sent, the field bus data link layer specifies how the network is jointly used and the individual devices obtain access to the network, and the application layer defines the application-specific services and data formats.
In field bus systems, digitally coded data are transmitted in discrete segments which are generally called data packets or messages. The number of users sharing a field bus medium is variable and, as a rule, is between 2 and 32 field devices, wherein logical link-ups of a number of field bus segments can also have three digit user numbers.
The data packets are transmitted on the field bus between the field devices and the controller. Each data packet can contain information with respect to the data link layer and information with respect to the application layer or with respect to other layers depending on the architecture of the system.
In a field bus system, errors can occur in different layers, starting with physical errors such as invalid signal levels, invalid signal forms and electromagnetic interference, via errors in the data link layer such as protocol violations up to errors in the application layer such as wrong configuration of the communication relations between the bus users. If the errors remain undetected, this can lead to the loss of critical information and to faulty operation of the field devices controlled by the field bus system. Errors can also impair the field bus system itself. To detect, analyze and eliminate such errors, the data traffic on the field bus is observed, therefore.
For functional checking and for fault finding in field bus systems, protocol analyzers such as message analyzers and bus monitors are used in the prior art. Such programs, which, as a rule, are PC-based, are connected to the field bus system to be examined via a bus interface such as an Ethernet port or a PROFIBUS interface. In the prior art, it is also known to connect to the field bus system independent testing devices in which the bus interface, a display and the test software are integrated in one device.
Protocol analyzers act passively, i.e. they observe the data traffic on the field bus but do not transmit themselves and record the data packets (messages) transmitted on the field bus. Although earlier versions of protocol analyzers were already capable of acquiring data in real time, the possibilities of analysis were limited. The data packets were acquired and stored initially and analyzed at a later time. Storage usually comprises the representation of the data packets in symbolic form in various protocol layers or levels of abstraction. Furthermore, the transmitting time of each data packet is logged by an associated time stamp. During the recording of the data packets, filter and trigger functions enable the volume of data to be reduced. Simple statistical evaluations, e.g. the indication of the addresses of the stations active on the bus (Live List) or the indication of error counters are also possible in the systems of the prior art.
From U.S. Pat. No. 5,442,639 and U.S. Pat. No. 5,796,721, protocol analyzers are also already known which are capable of analyzing the field bus traffic in real time. These systems, too, operate on the basis of filters, U.S. Pat. No. 5,796,721 describing a bus monitor which can allow the data packets from a field bus to pass through a number of filters. The filtered packets can be represented in real time. According to this document, the filter parameters can be varied in operation.
FIG. 2 shows by way of example the representation of the message traffic in a protocol analyzer of the prior art without any filtering. From left to right, a message number, a time stamp, source and destination address and message type or communication service are displayed. Clicking twice on a message causes it to be displayed in detail and to be decoded. The volume of data can be reduced by applying filters, as mentioned above.
The protocol analyzers of the prior art have a number of disadvantages which are represented briefly in the text which follows:                In industrial applications, there is demand for shorter and shorter response times which leads to ever increased transmission rates and thus to ever larger volumes of data which are transmitted on the field bus. A recording accurate to the bit position thus leads to data volumes of many megabytes per second which can only be handled with difficulty. For example, a PROFIBUS system generates about 1 Mbyte of message data per second at a transmission rate of 12 Mbit/sec.        Due to the large volumes of data, the bus traffic can only be recorded section by section. As an alternative, it must be filtered in advance in accordance with particular criteria. As a result, data packets may, under certain circumstances, be missing which are only transmitted at particular intervals such as at the bus or device start-up or which are filtered out due to preset filter criteria. However, such data packets can contain essential information for the interpretation of the field bus traffic.        In the known system, the user must interpret the messages. For this purpose, he needs detailed knowledge about the communication protocol of the field bus system which, as a rule, is not available to the operator of a field bus system.        Data packets can frequently be interpreted correctly only in the context of the states of the transmitting and receiving bus user. However, this context is known neither to the protocol analyzer nor to the user. This problem can be illustrated by means of the example of a chess game performed via mail. The significance of the chess move described in an arbitrarily selected letter only becomes apparent to the person who knows the state of the game. Thus, the situation on the chessboard must be known for a reasonable interpretation. In the same manner, a reasonable interpretation of the communication protocol of the field bus system can only be carried out with knowledge of the states of the connected users. This knowledge is largely lacking in the known protocol analyzers.        As a rule, traditional bus monitors with message recording are used only when an error has occurred and is to be analyzed. It is frequently difficult then to reproduce the error. In addition, correction of the bus monitor requires an intervention in the system which is frequently rejected by the system operator.        
It is, therefore, an object of the invention to specify a diagnostic procedure and a diagnostic facility for a field bus system which allows a continuous and on-line state analysis of the data traffic of the field bus system.